1 //==-- llvm/Support/ThreadPool.cpp - A ThreadPool implementation -*- C++ -*-==// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This file implements a crude C++11 based thread pool. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #include "llvm/Support/ThreadPool.h" 15 16 #include "llvm/Config/llvm-config.h" 17 #include "llvm/Support/raw_ostream.h" 18 19 using namespace llvm; 20 21 #if LLVM_ENABLE_THREADS 22 23 // Default to std::thread::hardware_concurrency 24 ThreadPool::ThreadPool() : ThreadPool(std::thread::hardware_concurrency()) {} 25 26 ThreadPool::ThreadPool(unsigned ThreadCount) 27 : ActiveThreads(0), EnableFlag(true) { 28 // Create ThreadCount threads that will loop forever, wait on QueueCondition 29 // for tasks to be queued or the Pool to be destroyed. 30 Threads.reserve(ThreadCount); 31 for (unsigned ThreadID = 0; ThreadID < ThreadCount; ++ThreadID) { 32 Threads.emplace_back([&] { 33 while (true) { 34 PackagedTaskTy Task; 35 { 36 std::unique_lock<std::mutex> LockGuard(QueueLock); 37 // Wait for tasks to be pushed in the queue 38 QueueCondition.wait(LockGuard, 39 [&] { return !EnableFlag || !Tasks.empty(); }); 40 // Exit condition 41 if (!EnableFlag && Tasks.empty()) 42 return; 43 // Yeah, we have a task, grab it and release the lock on the queue 44 45 // We first need to signal that we are active before popping the queue 46 // in order for wait() to properly detect that even if the queue is 47 // empty, there is still a task in flight. 48 { 49 ++ActiveThreads; 50 std::unique_lock<std::mutex> LockGuard(CompletionLock); 51 } 52 Task = std::move(Tasks.front()); 53 Tasks.pop(); 54 } 55 // Run the task we just grabbed 56 #ifndef _MSC_VER 57 Task(); 58 #else 59 Task(/* unused */ false); 60 #endif 61 62 { 63 // Adjust `ActiveThreads`, in case someone waits on ThreadPool::wait() 64 std::unique_lock<std::mutex> LockGuard(CompletionLock); 65 --ActiveThreads; 66 } 67 68 // Notify task completion, in case someone waits on ThreadPool::wait() 69 CompletionCondition.notify_all(); 70 } 71 }); 72 } 73 } 74 75 void ThreadPool::wait() { 76 // Wait for all threads to complete and the queue to be empty 77 std::unique_lock<std::mutex> LockGuard(CompletionLock); 78 // The order of the checks for ActiveThreads and Tasks.empty() matters because 79 // any active threads might be modifying the Tasks queue, and this would be a 80 // race. 81 CompletionCondition.wait(LockGuard, 82 [&] { return !ActiveThreads && Tasks.empty(); }); 83 } 84 85 std::shared_future<ThreadPool::VoidTy> ThreadPool::asyncImpl(TaskTy Task) { 86 /// Wrap the Task in a packaged_task to return a future object. 87 PackagedTaskTy PackagedTask(std::move(Task)); 88 auto Future = PackagedTask.get_future(); 89 { 90 // Lock the queue and push the new task 91 std::unique_lock<std::mutex> LockGuard(QueueLock); 92 93 // Don't allow enqueueing after disabling the pool 94 assert(EnableFlag && "Queuing a thread during ThreadPool destruction"); 95 96 Tasks.push(std::move(PackagedTask)); 97 } 98 QueueCondition.notify_one(); 99 return Future.share(); 100 } 101 102 // The destructor joins all threads, waiting for completion. 103 ThreadPool::~ThreadPool() { 104 { 105 std::unique_lock<std::mutex> LockGuard(QueueLock); 106 EnableFlag = false; 107 } 108 QueueCondition.notify_all(); 109 for (auto &Worker : Threads) 110 Worker.join(); 111 } 112 113 #else // LLVM_ENABLE_THREADS Disabled 114 115 ThreadPool::ThreadPool() : ThreadPool(0) {} 116 117 // No threads are launched, issue a warning if ThreadCount is not 0 118 ThreadPool::ThreadPool(unsigned ThreadCount) 119 : ActiveThreads(0) { 120 if (ThreadCount) { 121 errs() << "Warning: request a ThreadPool with " << ThreadCount 122 << " threads, but LLVM_ENABLE_THREADS has been turned off\n"; 123 } 124 } 125 126 void ThreadPool::wait() { 127 // Sequential implementation running the tasks 128 while (!Tasks.empty()) { 129 auto Task = std::move(Tasks.front()); 130 Tasks.pop(); 131 #ifndef _MSC_VER 132 Task(); 133 #else 134 Task(/* unused */ false); 135 #endif 136 } 137 } 138 139 std::shared_future<ThreadPool::VoidTy> ThreadPool::asyncImpl(TaskTy Task) { 140 #ifndef _MSC_VER 141 // Get a Future with launch::deferred execution using std::async 142 auto Future = std::async(std::launch::deferred, std::move(Task)).share(); 143 // Wrap the future so that both ThreadPool::wait() can operate and the 144 // returned future can be sync'ed on. 145 PackagedTaskTy PackagedTask([Future]() { Future.get(); }); 146 #else 147 auto Future = std::async(std::launch::deferred, std::move(Task), false).share(); 148 PackagedTaskTy PackagedTask([Future](bool) -> bool { Future.get(); return false; }); 149 #endif 150 Tasks.push(std::move(PackagedTask)); 151 return Future; 152 } 153 154 ThreadPool::~ThreadPool() { 155 wait(); 156 } 157 158 #endif 159
